Lubricating oil for a shock absorber in automobiles is employed mainly for damping vibration in order to attain optimum attenuation force and maintain driving stability.
Hitherto, lubricating oils for a shock absorber in automobiles have exhibited enhanced vibration damping effect through reducing the friction at a sliding interface between an oil seal and a piston rod, a piston rod and a guide bush, a piston band and a cylinder, etc. in a shock absorber (see, for example, Patent Documents 1 and 2).
However, due to recent construction of well-maintained roads such as highway networks, automobile drivers more frequently have the opportunity for high-speed driving. During such high-speed driving, micro-vibration is often generated through tires and in some cases impairs driving stability. Under such micro-vibration conditions, damping force is difficult to obtain. Therefore, it has been revealed that such micro-vibration cannot be suppressed by a conventionally developed shock-absorber lubricating oil which reduces friction.
Some subsequent studies have revealed that the micro-vibration can be suppressed through enhancing the frictional force between an oil seal and a piston rod, resulting in enhanced-level driving stability during high-speed driving (see, for example, Patent Document 3).
However, a shock-absorber lubricating oil which enhances the frictional force between an oil seal and a piston rod also increases the friction coefficient at a sliding interface other than the interface between an oil seal and a piston rod; e.g., the interface between a piston rod and a guide bush. As a result, when an automobile employing such a shock absorber oil travels on a manhole of a road (i.e., a road having small steps), a load lateral to the shock absorber (normal to the piston rod) is applied, and the friction coefficient between a piston rod and a guide bush increases. In this case, the thus-generated vibration cannot be damped, thereby impairing riding comfort.
Riding comfort is also varied by the foaming property of the shock absorber oil. Specifically, even when a shock absorber oil has an appropriate friction coefficient, when a large amount of foam is generated in the oil, the intrinsic performance of the oil cannot be attained, thereby impairing riding comfort.
Thus, in order to solve these problems, there is demand for a shock absorber lubricating oil which enhances the frictional force between an oil seal and a piston rod; which can reduce the friction coefficient at the other sliding interfaces, such as between a piston rod and a guide bush; and which suppresses foaming.
Meanwhile, since a lubricating oil for automobile shock absorbers is also employed in a cold district, high flowability must be also ensured at low temperature. However, in general, improvement in low-temperature flowability may impair riding comfort. Therefore, the low-temperature flowability must be improved while riding comfort is maintained.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. Hei 5-255683    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2000-192067            Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2003-147379        